1. Field
The embodiment relates to a light emitting device, a method of fabricating the light emitting device, a light emitting device package, and a lighting system.
2. Background
A light emitting device (LED) is a device having a characteristic of converting electrical energy into light energy. For example, the LED may represent various colors by adjusting the compositional ratio of compound semiconductors.
A LED according to the related art includes an N type GaN layer on a sapphire substrate, a multiple quantum well structure including an InGaN quantum well and a GaN quantum barrier, and a P type GaN layer, and emits light as electrons supplied from the N type GaN layer are combined with electrons supplied from the P type GaN layer in an active layer.
Although internal quantum efficiency of the LED approximates 100%, external quantum efficiency is only in the range of 2% to 30% since the total internal reflection, which is a physical phenomenon caused by a great difference in refractive index between a semiconductor and the air, prevents light, which is emitted from the active layer in the LED, from being extracted to the outside
For example, since a compound semiconductor, such as GaP or GaAs, has a refractive index of about 3.5 or 3.6, and the air has a refractive index of 1, a critical angle allowing the light emitted from the active layer to exit to the outside is about 16°. In the case of a GaN-based semiconductor device, since the refractive index of GaN is about 2.5, and the refractive index of the air is 1, the critical angle is about 23°. In other words, the light emitted from the active layer may exit to the air only if an incident angle of light onto a top surface of the active layer is less than 16° or 23°.
Accordingly, there are suggested schemes of highly transmitting photons to the external air by artificially making the surface of a semiconductor layer rough in order to improve external extraction efficiency of the photons.